Python – How to output list of floats to a binary file in Python

file-iopython

I have a list of floating-point values in Python:

floats = [3.14, 2.7, 0.0, -1.0, 1.1]

I would like to write these values out to a binary file using IEEE 32-bit encoding. What is the best way to do this in Python? My list actually contains about 200 MB of data, so something "not too slow" would be best.

Since there are 5 values, I just want a 20-byte file as output.

Best Solution

Alex is absolutely right, it's more efficient to do it this way:

from array import array
output_file = open('file', 'wb')
float_array = array('d', [3.14, 2.7, 0.0, -1.0, 1.1])
float_array.tofile(output_file)
output_file.close()

And then read the array like that:

input_file = open('file', 'rb')
float_array = array('d')
float_array.fromstring(input_file.read())

array.array objects also have a .fromfile method which can be used for reading the file, if you know the count of items in advance (e.g. from the file size, or some other mechanism)

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Python – How to check if a list is empty

if not a:
  print("List is empty")

Using the implicit booleanness of the empty list is quite pythonic.

Python – How to check whether a file exists without exceptions

If the reason you're checking is so you can do something like if file_exists: open_it(), it's safer to use a try around the attempt to open it. Checking and then opening risks the file being deleted or moved or something between when you check and when you try to open it.

If you're not planning to open the file immediately, you can use os.path.isfile

Return True if path is an existing regular file. This follows symbolic links, so both islink() and isfile() can be true for the same path.

import os.path
os.path.isfile(fname)

if you need to be sure it's a file.

Starting with Python 3.4, the pathlib module offers an object-oriented approach (backported to pathlib2 in Python 2.7):

from pathlib import Path

my_file = Path("/path/to/file")
if my_file.is_file():
    # file exists

To check a directory, do:

if my_file.is_dir():
    # directory exists

To check whether a Path object exists independently of whether is it a file or directory, use exists():

if my_file.exists():
    # path exists

You can also use resolve(strict=True) in a try block:

try:
    my_abs_path = my_file.resolve(strict=True)
except FileNotFoundError:
    # doesn't exist
else:
    # exists

Best Solution

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